System and process for calibrating pyrometers in thermal processing chambers

ABSTRACT

A method and system for calibrating temperature measurement devices, such as pyrometers, in thermal processing chambers are disclosed. According to the present invention, the system includes a calibrating light source that emits light energy onto a substrate contained in the thermal processing chamber. A light detector then detects the amount of light that is being transmitted through the substrate. The amount of detected light energy is then used to calibrate a temperature measurement device that is used in the system.

BACKGROUND OF THE INVENTION

A thermal processing chamber as used herein refers to a device thatrapidly heats objects, such as semiconductor wafers. Such devicestypically include a substrate holder for holding one or moresemiconductor wafers and an energy source for heating the wafers, suchas heating lamps and/or an electrical resistance heater. During heattreatment, the semiconductor wafers are heated under controlledconditions according to a preset temperature regime.

Many semiconductor heating processes require a wafer to be heated tohigh temperatures so that various chemical and physical transformationscan take place as the wafer is fabricated into a device. During rapidthermal processing, for instance, semiconductor wafers are typicallyheated by an array of lights to temperatures from about 300° C. to about1,200° C., for times that are typically less than a few minutes. Duringthese processes, one main goal is to heat the wafers as uniformly aspossible.

During the rapid thermal processing of a semiconductor wafer, it isdesirable to monitor and control the wafer temperature. In particular,for all of the high temperature wafer processes of current andforeseeable interest, it is important that the true temperature of thewafer be determined with high accuracy, repeatability and speed. Theability to accurately measure the temperature of a wafer has a directpayoff in the quality and size of the manufactured integrated circuit.

One of the most significant challenges in wafer heating systems is theability to measure accurately the temperature of substrates during theheating process. In the past, various means and devices for measuringthe temperature of substrates in thermal processing chambers have beendeveloped. Such devices include, for instance, pyrometers, thermocouplesthat directly contact the substrate or that are placed adjacent to thesubstrate, and the use of laser interference.

In order to use pyrometers in a thermal processing chamber, thepyrometers generally need to be calibrated. Consequently, variouscalibration procedures currently exist to align the temperature readingsof the pyrometers with an absolute and accurate temperature reference.The current state of the art and the most widely used method tocalibrate pyrometers in thermal processing chambers is to place in thechambers a semiconductor wafer having a thermocouple embedded in thewafer. The temperature measurements taken from the thermocouple arecompared with the temperature readings received from the temperaturemeasuring devices and any discrepancy is calibrated out.

Although this method is well suited to calibrating temperature measuringdevice, such as pyrometers, it requires a substantial amount of time tocalibrate the instruments. As such, a need currently exists for a methodof calibrating pyrometers in thermal processing chambers very rapidlywithout creating a substantial amount of down time. In particular, aneed exists for a method of calibrating pyrometers in thermal processingchambers without having to open the chamber, in order to maintainchamber integrity and purity. A need also exists for a simple method forcalibrating pyrometers in thermal processing chambers that can be usedroutinely as a regular check to verify that the optical pyrometry systemis properly functioning.

SUMMARY OF THE INVENTION

The present invention is directed to a process for calibrating atemperature measurement device in a thermal processing chamber. Theprocess includes the steps of placing a calibration wafer within thethermal processing chamber. Light energy is emitted from a calibratinglight source onto the calibration wafer while the wafer is being heatedwithin the thermal processing chamber. For instance, the wafer can beheated using light energy and/or by using an electrical resistanceheater. The amount of light energy emitted from the calibrating lightsource that is transmitted through the calibration wafer is detected.The temperature of the calibration wafer is then determined based uponthe amount of transmitted light that is detected.

From this information, the temperature measurement device containedwithin the thermal processing chamber can be calibrated. The temperaturemeasurement device can be, for instance, one or more pyrometers, one ormore thermocouples, or any other suitable temperature measurementdevice.

During the process, the light energy that is transmitted through thecalibration wafer is detected at one or more specific wavelengths. Ingeneral, the wavelength can be in the infra-red range. For instance, thedetected wavelength can be from about 1 micron to about 2 microns. Inone embodiment, transmitted light is detected at several wavelengthssimultaneously. In an alternative embodiment, however, a firstwavelength is used to detect the amount of light transmitted through thewafer at lower temperatures, while a second wavelength is used todetermine the amount of light being transmitted through the wafer athigher temperatures. For example, the first wavelength can be shorterthan the second wavelength. The second wavelength can be used todetermine the temperature of the wafer at temperatures greater thanabout 700° C.

The calibrating light source used in the present invention can be acoherent light source or an incoherent light source. An example of acoherent light source is, for instance, a laser. Examples of incoherentlight sources are, for instance, a tungsten halogen lamp or a lightemitting diode.

The calibration wafer used in the process of the present invention canvary depending upon the particular application. In one embodiment, thecalibration wafer is a silicon wafer. In order to reduce interferenceeffects and to maximize the amount of light transmitted through thecalibration wafer, the calibration wafer can include anti-reflectivecoatings applied to one or both surfaces of the wafer. For measurementsat higher temperatures, the calibration wafer can also include thinareas where the transmission measurements are taken and the temperaturemeasurements devices are calibrated.

When the calibration wafer includes a thin area, the thin area can beproduced according to various methods. For instance, in one embodiment,the calibration wafer can define an opening. The thin area can comprisea thin member placed over the opening. Alternatively, the thin area canbe integral with the remainder of the wafer.

In some embodiments, the thin area may produce temperature gradientsduring heating as a result of differences in thermal characteristics.Consequently, a coating can be placed over the calibration wafer that isdesigned to reduce the differences in thermal characteristics betweenthe thin area and the remainder of the wafer.

The coating can be made from single or multi-layered films. The coatingcan contain, for instance, silicon, polysilicon, and/or silicon nitride.In one particular embodiment, the thin area can be filled with a fillmember for reducing thermal mass differences. The fill member can bemade from quartz or aluminum oxide such as sapphire.

It should be understood that the calibration wafer can be made fromvarious materials. In one embodiment, for instance, the calibrationwafer can be made from a substantially opaque material. Opaque materialsare useful when calibrating pyrometers at lower temperatures. In thisembodiment, the calibration wafer can further include areas made fromtransmissive materials, such as silicon. The silicon areas can be usedto measure transmission, while the opaque areas can be aligned with thepyrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended figures in which:

FIG. 1 is a side view of one embodiment of a system for calibratingtemperature sensing devices in accordance with the present invention;

FIG. 2 is a graph illustrating the amount of light that is transmittedthrough a silicon wafer at different temperatures and wavelengths;

FIG. 3 is a side view of one embodiment of a system made in accordancewith the present invention;

FIG. 4 is a perspective view of an alternative embodiment of a systemmade in accordance with the present invention;

FIG. 5 is another alternative embodiment of a system made in accordancewith the present invention;

FIGS. 6 through 21 are different embodiments of calibration wafers madein accordance with the present invention; and

FIG. 22 is a cross-sectional diagrammatical view showing multiplereflections of light in between the outside surfaces of a wafer.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

The present invention is directed to a method and to a system for moreaccurately determining and controlling the temperature of an object,particularly a semiconductor wafer in a thermal processing chamberduring heat treatment. More particularly, the present invention isdirected to a method and system for calibrating temperature measuringdevices contained within thermal processing chambers so that the thermalprocessing chamber will operate more repeatably and accurately. Forinstance, it is important that temperature sensing devices containedwithin thermal processing chambers accurately measure the temperature ofsemiconductor wafers as they are being heated. In this regard, thetemperature sensing devices should be calibrated to ensure that they areaccurately monitoring the temperature of the wafer.

In general, the method of the present invention for calibratingtemperature sensing devices, particularly radiation sensing devices suchas pyrometers, includes the step of placing in a thermal processingchamber a calibration wafer. A calibrating light source placed in thechamber is configured to emit light energy onto the calibration wafer ata known wavelength. Positioned on the opposite side of the wafer is alight detector that detects the amount of light being transmittedthrough the wafer by the calibrating light source. The temperature ofthe wafer is then calculated based on the amount of light that istransmitted through the wafer. This information is used to calibrate atemperature sensing device contained within the chamber which is usedduring normal wafer processing.

The present invention permits automatic calibration of a temperaturemeasurement system through an automated procedure that is performed on asemiconductor wafer processing system. The temperature calibrationmethod is based on an in-situ measurement of the transmission ofinfra-red light through the semiconductor wafer. The wafer processingapparatus incorporates a device for measuring a signal arriving from theinfra-red light transmitted by the wafer as well as a temperaturemeasurement system, such as a pyrometer system that is used duringnormal processing for temperature measurement and control. Knowledge ofthe temperature dependence of the optical properties of the wafer usedfor the calibration procedure is combined with the measured infra-redtransmission signal to deduce the wafer temperature. The wafertemperature is used to calibrate a pyrometer system or other temperaturemeasurement device.

A light source on one side of a calibration wafer is modulated inintensity, and a detector on the opposite side of the wafer detects asignal proportional to the amount of light transmitted through thewafer. The system incorporates a method for selecting the wavelength ofthe observed radiation. The transmitted signal depends on opticalabsorption within the wafer, which is a function of temperature. As aresult, the wafer temperature can be deduced from the transmitted lightsignal.

The infra-red transmission measurement is arranged so that it occurs ata position on the wafer that is within the field of view or close to thefield of view (within a few centimeters) of the pyrometer or othertemperature measurement device. If the system allows for wafer rotation,it is also possible for the infra-red transmission measurement to beperformed at the same radius or close to the same radius as the field ofview of the pyrometer or temperature measurement device beingcalibrated. Provided that the wafer rotation is at a rate that is fastenough to create an azimuthally symmetric temperature distribution, thetemperature sampled in the infra-red measurement is the same as thatsampled by the pyrometer at the same particular radius. In oneembodiment, the same fiber or optics that lead to the light detector fordetermining the amount of infra-red light being transmitted through thewafer can lead to the pyrometer, such as by splitting the signaloptically.

Calibration of the temperature measurement device can be performedthrough an automated procedure where wafers are loaded automatically anda calibration recipe heats the wafer through a prescribedtemperature-time cycle and data is acquired by the infra-redtransmission system and pyrometer system. The procedure can include astage where the measurement is taken with the wafer at a temperaturewhere its transmission is known and it is not highly sensitive totemperature. This allows the signal to be corrected so that thetransmission measurement is not highly sensitive to changes in theoptical characteristics of the transmission measurement system or thecalibration wafer.

The transmission signal is interpreted as a temperature by an algorithmthat takes account of the properties of the wafer being used for thecalibration. The algorithm can accept information regarding thethickness of the wafer, the wavelength of light being used forcalibration and other parameters that help improve the accuracy of thetemperature value deduced from the transmission signal.

The process for calibrating the temperature measurement device can alsoinclude a step in which measurements using the transmission system occurbefore a wafer is loaded into the thermal processing chamber. The ratioof the transmission signal with a wafer in place as compared to thatwithout a wafer provides an estimate of the transmissivity of the wafer.The measured transmissivity of the wafer can then be used to ensure thatthe correct wafer was loaded and to test for degradation of the wafer.The transmission signal level when there is no wafer present is also auseful indicator of the status of the optical system for transmissionmeasurements.

The method and system of the present invention offer various advantagesand benefits. For instance, the present invention offers a relativelysimple method for quickly calibrating the radiation sensing devices. Thecalibrations can be performed automatically between wafers or wafer lotsby placing a calibration wafer into the thermal processing chamber whendesired. Further, the calibration wafer can be loaded and removed fromthe thermal processing chamber using the same mechanism that moves andtransports the wafers.

The present invention allows for the calibration of radiation sensingdevices in thermal processing chambers according to a relatively simplemethod that does not require substantial interference with the operationof the chamber. The present invention can be used to calibrate single ormulti-point pyrometric systems. Further, the method and system of thepresent invention permit calibration without having to incorporate athermocouple instrumented wafer as was done in the past.

Referring now to FIG. 3, one embodiment of a simplified diagram of thepresent invention is illustrated. As shown, the system includes asemiconductor wafer 14 and a temperature measurement device 27, such asa pyrometer, that is normally used to monitor the temperature of thewafer. In order to calibrate the temperature measurement device 27, thesystem further includes a calibrating light source 23 which emitsinfra-red light onto the wafer 14 at a particular location. On theopposite side of the wafer is a light detector 42 which senses theamount of infra-red light that is being transmitted through the wafer14.

Measurements of the transmissivity of the wafer can be used to determinethe temperature because the absorption coefficient of silicon, α(λ,T) isa function of the wavelength of the radiation, λ, and the temperature T.The following discussion includes the mathematics relating thetransmission measurements to α(λ,T).

A general wafer has various properties that need to be considered. Thetwo surfaces of the wafer may have different reflectivities andtransmissivities. Furthermore, the reflectivities of the surfaces may bedifferent for radiation incident on them from outside the wafer, or fromwithin the wafer. When a wafer is semi-transparent, multiple reflectionsof the various beams of energy propagating within the wafer affect itsapparent reflectivity R*(λ,T) and transmissivity S*(λ,T) as observedfrom outside the wafer. The later quantities can be measured directly byoptical measurements. In the calibration method, S*(λ,T) can bemeasured, because this quantity is the most sensitive to temperature(although R*(λ,T) can also be used for temperature measurements). Insome texts, R*(λ,T) is called the reflectance and S*(λ,T) thetransmittance.

In the discussion below, T_(t) is the transmissivity of the top surfaceof the wafer, T_(b) is the transmissivity of the bottom surface of thewafer, R_(ts) is the reflectivity of the top surface of the wafer forradiation incident on it from within the substrate and R_(bs) is thereflectivity of the bottom surface of the wafer for radiation incidenton it from within the substrate. In general, if the incident radiationis not at normal incidence, all of the properties will be functions ofthe plane of polarization of the radiation. The quantity A is theattenuation in intensity experienced by a ray passing through thesubstrate, which is given byA=exp(−α(λ,T)d/cos θ)  (1)

Where d is the thickness of the substrate and θ is the internal angle ofpropagation. The latter angle is the angle between the direction of theray and the normal to the wafer surface. The apparent transmissivity ofthe wafer is then given by the expression

$\begin{matrix}{{S^{*}\left( {\lambda,T} \right)} = \frac{{AT}_{t}T_{b}}{1 - {A^{2}R_{ts}R_{bs}}}} & (2)\end{matrix}$

The signal measured at the photodetector, V(T), in the apparatus isdirectly proportional to this quantity and to the intensity of theradiation incident on the wafer from the illuminating light source, I₀.The relationship isV(T)=CI ₀ S*(λ,T)  (3)

Where C is a constant that is affected by the optics and electronics,but that does not vary with wafer temperature. In order to deduce thewafer temperature, the effects of the unknown quantities, including I₀and C, from the measurements should be removed. This can be done in anumber of ways. The optical properties of the wafer surfaces (such astheir reflectivities and transmissivities) should also be taken intoaccount. A “normalization” procedure can take care of these aspects. Forexample, the following are two approaches:

(a) Normalize to the case where the wafer is not present in the system:

In this case the transmissivity of the system becomes 1, because thereis no wafer present to absorb or reflect any radiation. A normalizationsignal V₀₀ can be measured which is given byV₀₀=CI₀  (4)

By dividing a “hot-wafer” signal V(T) by V₀₀, the effects of C and I₀can be removed. The effects of the other quantities in S*(λ,T) stillmust be accounted for however. That can be done by knowing the opticalproperties of the wafer being used, including T_(t), T_(b), R_(bs) andR_(ts). In this regard, the wafer being used in the calibrationprocedure can be selected so that these quantities can be determined bymaking calculations based on the known film coatings on the surfaces ofthe wafer. The calculations can also be augmented by various opticalmeasurements that can be made under controlled conditions. Theabsorption coefficient can then be obtained from the measured value ofS*(λ,T) by solving for A in equation 1. Once A is known, it is possibleto correct for the effect of the wafer thickness (which has beenmeasured) and deduce α(λ,T).

Since the function α(λ,T) is known, T can be deduced. This procedurealso allows to correct for variations in the wavelength of the lightsource. The exact method of implementing this procedure could varydepending on the application. For instance, as one example one couldcreate a set of matrices for S*(λ,T) with wafer thickness and sourcewavelength as parameters and T as a variable and then numericallyidentify a best value for T to fit the measured value of S*(λ,T).

The quantities T_(t), T_(b), R_(bs) and R_(ts) may themselves havetemperature and wavelength dependences. For accuracy this effect couldbe taken into account in the analysis by explicitly including the knownbehaviour of the wafer surfaces. Again, the corrections are possible byusing standard optical analysis to calculate T_(t), T_(b), R_(bs) andR_(ts) as functions of wavelength and temperature.

(b) Normalize to the case where the wafer is in a “cool” state:

In this case the normalization signal, V₀₁, is collected after thecalibration wafer has been loaded into the system. V₀₁ is given byV ₀₁ =CI ₀ S*(λ,T _(cool))  (5)where S*(λ,T_(cool)) is the wafer transmissivity at the “cool”temperature. That temperature would normally be any temperature wherethe absorption in the wafer at the calibration wavelength is negligible.In that case A˜1, and we can write:

$\begin{matrix}{V_{01} = {{CI}_{0}\frac{T_{t}T_{b}}{1 - {R_{ts}R_{bs}}}}} & (6)\end{matrix}$

Now if a “hot-wafer” signal V(T) is normalized using V₀₁ we obtain theexpression

$\begin{matrix}{\frac{V(T)}{V_{01}} = {\frac{{AT}_{t}T_{b}}{1 - {A^{2}R_{ts}R_{bs}}} \cdot \frac{1 - {R_{ts}R_{bs}}}{T_{t}T_{b}}}} & (7)\end{matrix}$

If it is assumed that T_(t) and T_(b) do not depend on temperature, thenthis simplifies to

$\begin{matrix}{\frac{V(T)}{V_{01}} = \frac{A\left( {1 - {R_{ts}R_{bs}}} \right)}{1 - {A^{2}R_{ts}R_{bs}}}} & (8)\end{matrix}$

Further simplification can occur by examining the magnitude of theproduct A²R_(ts)R_(bs) which appears in expression 8. This term isusually very small for two reasons. Firstly, for good temperaturesensitivity there must be reasonably strong absorption within the waferso A should be significantly below 1. Secondly, a wafer can be selectedhaving rather small values for the reflectivity of the wafer surface atthe transmission measurement wavelength. This can be done to maximizethe transmitted light signal. Another practical point is that theinterference effect observed when coherent light sources are used forillumination causes some problems in the measurements. By having a verysmall value for either R_(ts) or R_(bs) the problems identified abovecan be reduced. Furthermore, even for a plain silicon wafer,Rts=Rbs=˜0.3, so R_(ts)R_(bs)˜0.09. Thus, the term R_(ts)R_(bs) isusually <<1. As a result, one can make the approximation of ignoringthis term, as compared to unity, in the denominator of expression 8.This simplification leads to a simple form for the signal whennormalized to the “cool” state

$\begin{matrix}{\frac{V(T)}{V_{01}} \cong {A\left( {1 - {R_{ts}R_{bs}}} \right)}} & (9)\end{matrix}$if it is assumed that R_(ts)R_(bs) is <<1 then the expression canfurther simplify to

$\begin{matrix}{{\frac{V(T)}{V_{01}} \cong A} = {\exp\left( {{- {\alpha\left( {\lambda,T} \right)}}d} \right)}} & (10)\end{matrix}$

The advantage of this approach is that, provided there is no significanttemperature dependence in T_(t) or T_(b), it is not necessary to knowthe optical properties of the wafer surfaces. This can make theinterpretation of the temperature more robust with respect to variationsin the properties of coatings on the wafer and the texture of thesurfaces.

The question of which normalization approach is better is driven bypracticalities. The advantages of the case where the measurement istaken without the wafer present is that one doesn't have to worry aboutinterference effects, which have their greatest impact in the “cold”state. The advantage of making the measurement with the wafer in the“cool” state is that there is automatic compensation for any smallfluctuations in the surface reflectivities, transmissivities or textureeffects.

The analysis described above is all based on the assumption that theapparent reflectivity and transmissivity can be calculated by adding upthe power contributions arising from the various rays undergoingmultiple reflections illustrated in FIG. 22. This assumption is validwhen there is no correlation between the electric fields associated withthose rays. That is the case when the detected radiation covers anoptical bandwidth which is a significant fraction (e.g. >0.001) of the“centre” wavelength, and the wafer is “optically thick”, meaning itsthickness corresponds to a large multiple (e.g. >100 times) of thewavelength of the light. These conditions correspond to the case whenthe light is regarded as “incoherent”.

In the case where the light source is “coherent”, typically theradiation being detected covers a very narrow range of wavelengthsand/or the wafer is optically “thin”. In that case there can be a strongcorrelation between the electric fields of the various rays propagatingwithin the wafer. If this is the case summing up the power contributionsdoes not reflect the correct physics. Instead, one needs to make avector summation of the electric and magnetic fields of theelectromagnetic waves associated with the rays undergoing the multiplereflections in order to obtain the correct apparent reflectivities andtransmissivities. Under these conditions, the effect of the changes inthe phase of the waves as they traverse the substrate becomes veryimportant. If the fields for two wave components are added together inphase the effect is known as constructive interference because theresultant field is enlarged, if they are exactly out of phase this isknown as destructive interference because the resultant field isreduced. These interference effects can have a major effect on theoptical properties and furthermore they make them extremely sensitive tothe phase-change that occurs, and hence to the wafer thickness, thewavelength of the radiation, the angle of incidence and the refractiveindex of the substrate.

This can be a problem, because it introduces very large variations inthe detected signals which are not related to changes in the α(λ,T), theabsorption coefficient of silicon. The problem is greatly reduced asabsorption increases and the wafer becomes opaque, because the multiplereflections are greatly attenuated and most of the transmitted lightintensity chiefly arises from light that passes through the substratedirectly, so interference effects have little impact. Various methodsare described below to mitigate problems arising from interferenceeffects.

From the ratio of transmission of a wafer at a relatively hightemperature in relation to transmission where absorption is negligible(see above equations) such as at low temperatures or when no wafer ispresent in the system, the temperature of the wafer can be calculated atthe higher temperatures. For instance, FIG. 2 is a graph showing thetransmission ratio measured for a silicon wafer having a diameter of 200mm and a thickness of about 725 microns at various wavelengths. Asshown, the wavelengths vary from 1.064 microns to 1.55 microns. As alsoshown, the transmission at the longer wavelengths of light can be usedto determine higher temperatures.

At the included wavelengths of light, however, temperature readingsabove about 850° C. become difficult because the transmitted lightsignal becomes small and is difficult to detect. In order to taketemperature readings even at higher temperatures, a calibration wafercan be used that is thinner than the 725 micron wafer used to generatethe results in FIG. 2. The use of thinner wafers to determinetemperatures above 850° C. will be discussed in more detail below.

When calibrating the temperature measurement device 27 as shown in FIG.3, it is desirable for the calibrating light source 23 and the lightdetector 42 to take readings as close as possible to the location on thewafer where the temperature is measured by the temperature measuringdevice 27. In other words, the calibration probe should be at a positioneither within the field-of-view of the pyrometer or near enough forthere to be very small temperature differences between the temperatureat the calibration probe and the temperature in the pyrometer'sfield-of-view. This can be quantified by referring to the thermaldiffusion length, L_(d), which is given by:

$L_{d} = \sqrt{\frac{k_{Si}d_{w}}{4\left( {ɛ_{f} + ɛ_{b}} \right)\sigma\; T^{3}}}$where k_(Si) is the thermal conductivity of silicon, d_(w) is the waferthickness, ε_(f) and ε_(b) are the total emissivities of the front andback surfaces respectively, σ is the Stefan-Boltzmann constant and T isthe wafer temperature.

For many applications, the calibration point should be within ˜L_(d) ofthe field-of-view of the pyrometer. As the temperature increases, thisdistance decreases. For example, at high temperatures such as greaterthan 1100° C., this distance can be as small as approximately 4 mm.Regardless, for many applications, the distance between the calibrationpoint and the pyrometer field-of-view should not exceed about 3 L_(d).For instance, in most applications, the distance between the location onthe wafer where the temperature is measured by the temperature measuringdevice and the location on the wafer where the light detector detectsthe transmitted light should be no greater than about 5 cm.

When the wafer is being rotated in the thermal processing chamber,however, accuracy can be maintained by detecting the transmitted lighton the same or near the same radius of the wafer as where thetemperature measurement device 27 measures the wafer temperature asillustrated in FIG. 4.

Referring to FIG. 1, one embodiment of a thermal processing systemgenerally 10 incorporating a system for calibrating temperature sensingdevices in accordance with the present invention is illustrated.

System 10 includes a processing chamber 12 adapted to receive substratessuch as semiconductor wafers for conducting various processes. Chamber12 is designed to heat the wafers at very rapid rates and undercarefully controlled conditions. Chamber 12 can be made from variousmaterials, including certain metals, glasses and ceramics. For instance,chamber 12 can be made from stainless steel or quartz.

When chamber 12 is made from a heat conductive material, preferably thechamber includes a cooling system. For instance, as shown in FIG. 1,chamber 12 includes a cooling conduit 16 wrapped around the perimeter ofthe chamber. Conduit 16 is adapted to circulate a cooling fluid, such aswater, which is used to maintain the walls of chamber 12 at a constanttemperature.

Chamber 12 can also include a gas inlet 18 and a gas outlet 20 forintroducing a gas into the chamber and/or for maintaining the chamberwithin a preset pressure range. For instance, a gas can be introducedinto chamber 12 through gas inlet 18 for reaction with the wafers. Onceprocessed, the gas can then be evacuated from the chamber using gasoutlet 20.

Alternatively, an inert gas can be fed to chamber 12 through gas inlet18 for preventing any unwanted or undesirable side reactions fromoccurring within the chamber. In a further embodiment, gas inlet 18 andgas outlet 20 can be used to pressurize chamber 12. A vacuum can also becreated in chamber 12 when desired, using gas outlet 20 or an additionallarger outlet positioned beneath the level of the wafer.

During processing, chamber 12, in one embodiment, can include asubstrate holder 15 designed to rotate wafers using a wafer rotationmechanism 21 as shown by the arrow located above the wafer 14. Rotatingthe wafer promotes greater temperature uniformity over the surface ofthe wafer and promotes enhanced contact between the wafer and any gasesintroduced into the chamber. It should be understood, however, thatbesides wafers, chamber 12 is also adapted to process optical parts,films, fibers, ribbons, and other substrates having any particularshape.

A heat source generally 22 is included in communication with chamber 12for heating the wafers during processing. In this embodiment, heatsource 22 includes a plurality of lamps or light sources 24, such astungsten-halogen lamps. Heat source 22 can include a reflector or set ofreflectors, for carefully directing thermal energy being emitted by theheat source onto the wafers so as to produce a very uniform wafertemperature. As shown in FIG. 1, lamps 24 are placed above the chamber.It should be understood, however, that lamps 24 may be placed at anyparticular location, such as below the wafer either alone or incombination with lamps 24.

The use of lamps 24 as heat source 22 can offer various advantages. Forinstance, lamps have much higher heating and cooling rates than otherheating devices, such as electrical elements or conventional furnaces.Lamps 24 create a rapid isothermal processing system that provideinstantaneous energy, typically requiring a very short and wellcontrolled start up period. The flow of energy from lamps 24 can also beabruptly stopped at any time. As shown in the figure, lamps 24 areequipped with a gradual power controller 25 that can be used to increaseor decrease the thermal energy being emitted by the lamps.

Besides using lamps 24 or in addition to using lamps 24 as the heatsource 22, the system 10 can include a heated susceptor 26 for heatingthe wafer 14. The susceptor 26, can be, for instance, an electricalresistance heater or an induction heater. In the system illustrated inFIG. 1, the susceptor 26 is placed below the wafer 14. Similar to theheating lamps, however, the susceptor 26 can be placed below the wafer,can be placed above the wafer, or the system can include multiplesusceptors that are place above and below the wafer.

Also contained within chamber 12 is a plurality of radiation sensingdevices generally 27. Radiation sensing devices 27 include opticalfibers or light pipes 28 which are, in turn, in communication with aplurality of corresponding light detectors 30. Optical fibers 28 areconfigured to receive thermal energy being emitted by a wafer present inthe chamber at a particular wavelength. The amount of sensed radiationis then communicated to light detectors 30 which generate a usablevoltage signal for determining the temperature of the wafer. In oneembodiment, each optical fiber 28 in combination with a light detector30 comprises a pyrometer.

As shown, system 10, includes a window 32 which separates the lamps 24from the chamber 12. In the embodiment illustrated, window 32 serves toisolate lamps 24 from the wafers and prevent contamination of thechamber.

As shown in FIG. 1, in accordance with the present invention, system 10further includes calibrating light source 23 and a light detector 42 inorder to calibrate radiation sensing devices 27. As described above,calibrating light source 23 emits light energy, particularly infra-redlight energy onto the wafer at a particular location. The light emittedfrom the calibrating light source 23 that transmits through the wafer isdetected by the light detector 42. From this information, thetemperature of the wafer can be determined in order to calibrate theradiation sensing devices.

As shown in FIG. 1, the calibrating light source 23 can be positionedamong the heating lamps 24 directly opposite the wafer 14.Alternatively, however, the calibrating light source 23 can bepositioned outside of the chamber or in an alternative location. In thisembodiment, light emitted from the calibrating light source can bedelivered to the wafer 14 using fiberoptics.

In order to accurately measure the amount of light energy beingtransmitted through the wafer, the light detector 42 can include a lightchannel 44 that directs the transmitted light to the light detector asshown in FIG. 1.

Calibrating light source 23 used in the system of the present inventioncan generally be any device that is capable of emitting light energy ata desired wavelength. For instance, calibrating light source 23 can bean incoherent light source or a coherent light source. Examples ofincoherent light sources include tungsten-halogen lamps, arc lamps,light emitting diodes, super-luminescent light emitting diodes, etc.Coherent light sources, on the other hand, include a solid state devicesuch as a laser diode, a super fluorescent fiber laser, other types oflasers, etc.

When it is necessary to calibrate the radiation sensing devices, acalibration wafer 14 is placed in the chamber and the above procedure iscarried out. In FIG. 1, only a single calibrating light source 23 andlight detector 42 are shown. It should be understood, however, that formost applications, the system will include multiple light detectors andcorresponding calibrating light sources for calibrating the variousradiation sensing devices.

Referring to FIG. 1, system 10 further includes a system controller 50which can be, for instance, a microprocessor. Controller 50 receivesvoltage signals from light detectors 30 that represent the radiationamounts being sampled at the various locations. Based on the signalsreceived, controller 50 is configured to calculate the temperature ofwafers contained in the chamber.

System controller 50 as shown in FIG. 1 can also be in communicationwith lamp power controller 25. In this arrangement, controller 50 cancalculate the temperature of a wafer, and, based on the calculatedinformation, control the amount of thermal energy being emitted by lamps24. In this manner, instantaneous adjustments can be made regarding theconditions within reactor 12 for processing the wafer within carefullycontrolled limits. As described above, in addition to the lamps 24 or asan alternative to the lamps 24, the system can further include asusceptor 26 as shown in FIG. 1. In this embodiment, the controller 50can also be used to control the amount of heat being emitted by thesusceptor. When both heating devices are present, the susceptor can becontrolled independently of the lamps or in conjunction with the lamps.(Or the system might not contain lamps, only a susceptor).

In one embodiment, controller 50 can also be used to automaticallycontrol other elements within the system. For instance, controller 50can be used to control the flow rate of gases entering chamber 12through gas inlet 18. As shown, controller 50 can further be used tocontrol the rate at which wafer 14 is rotated within the chamber.

In accordance with the present invention, system controller 50 can alsobe used to automatically calibrate radiation sensing devices 27. Forinstance, controller 50 can also be in communication with calibratinglight source 23 and light detection 42. In this manner, controller 50can be used to control when calibrating light source 23 emits light andthe amount of light that is emitted. Controller 50 can also beconfigured to receive information from the light detector 42 fordetermining the temperature of a calibration wafer and thereaftercalibrating the radiation sensing devices 27 based upon the determinedtemperature.

Various embodiments of the present invention will now be discussed. Inparticular, first operation of the present invention using an incoherentlight source will be described followed by a discussion of using acoherent light source. Subsequently, various embodiments of acalibration wafer will be described. Finally, methods for carrying outthe present invention will be discussed.

Calibrating Light Source

A. Incoherent Light Source

As stated, in one embodiment, the calibrating light source 23 can be anincoherent lamp, such as a tungsten-halogen lamp or a light emittingdiode. For example, in one embodiment, the calibrating light source canbe a super-luminescent light emitting diode. Some incoherent lightsources have the advantage that they can be used as a light source fortransmission measurements at several wavelengths as desired. Forexample, some incoherent light sources emit a wide range of wavelengths,while others, such as a light emitting diode, emit relatively narrowranges of wavelengths. When using an incoherent lamp, the light detector42 can include a single detector for detecting a single wavelength orcan include a detector array that measures the transmitted signal atseveral wavelengths simultaneously.

Besides including a single calibrating light source 23 as shown in FIG.1, it should also be understood that multiple calibration light sourcescan be present in the system of the present invention. Alternatively, asingle calibrating light source can be used that is placed incommunication with multiple optic fibers for transmitting light onto thewafer 14 at multiple positions.

In one embodiment, the output of the calibrating light source can bechopped, for example, by a mechanical chopper or can be choppedelectronically. Emitting the light from the calibrating light sourceonto the wafer at intervals can assist in determining the amount ofbackground stray light that is present by making a measurement when thecalibrating light source is emitting light and when the calibratingsource is not emitting light.

One embodiment of a system made according to the present invention usingan incoherent light source as the calibrating light source 23 is shownin FIG. 5. In this embodiment, the light source 23 transmits a beam oflight through a calibrating wafer 14, which is detected by a lightdetector 42. The light detector 42 can be, for instance, a photodetectoror any other suitable device.

In this embodiment, the lamp 23 is used in conjunction with collimatingoptics 60 for focusing the light onto a particular location of the wafer14. In order to measure the light being transmitted through the wafer 14and in order to eliminate stray light, the system can include one ormore apertures 62 that define the field of view for the light detector42. A spectral filter 64 can also be included in order to better definethe wavelength band that is to be detected.

Although the embodiment shown in FIG. 5 has been described in relationto the use of an incoherent light source, it should be understood thatthe use of collimating optics and generally the configurationillustrated in FIG. 5 can be used with all different types of lightsources, whether the light sources are coherent or incoherent.

B. Coherent Light Sources

As an alternative to using an incoherent light source, the calibratinglight source can also be a coherent light source, such as a laser whichcan provide relatively high power in a narrow wavelength range. Oneparticular example of a coherent light source is a semiconductor laser.Such light sources are easily modulated electrically. Coherent lightsources, such as lasers, have a very narrow emission spectrum. Althoughthis can provide some advantages, having a narrow emission spectrum canalso pose potential problems. In particular, the system may be moresensitive to interference effects when light being emitted by thecoherent light source is reflected between the two surfaces of thewafer. In particular, the transmitted signal that ultimately reaches thelight detector 42 may be more sensitive to the wafer thickness and itsrefractive index than when using an incoherent light source. Severalmeasures, however, can be taken to counteract this effect.

For instance, in one embodiment, a coherent light source can be usedthat has a multiple wavelength emission spectrum or a collection ofnarrow emission spectrums. In another embodiment, the calibrating wafercan include at least one rough surface, although this may have theundesirable effect of scattering some of the light from the lightsource. In still another embodiment of the present invention, the pathlength of the light being emitted by the calibrating light source can bechanged, for instance, by making small changes in the wafer thickness.Specifically, the thickness of the wafer can be varied such as bygradually decreasing in the areas where the transmission measurementsare to occur. In another embodiment, the calibration wafer can includeanti-reflective coatings as will be described in more detail below.

When the wafer is rotated during heating, it is possible to exploit therotation in order to decrease the effect of interference within thewafer. One approach is to rotate the wafer and measure the transmittedlight signal. As the wafer rotates, the thickness of the wafer probed bythe infra-red beam will alter slightly and the condition forinterference will change, causing oscillations in the transmitted lightintensity. By collecting the signal over at least one rotation, theeffect of interference can be averaged out to obtain a more reliablevalue for the transmission.

Another approach involves placing an optical element (such as a groundglass plate) in the path of the coherent light source such that thelight beam loses its spatial coherence. By altering the phase of theelectromagnetic field oscillations across the profile of the beam andthen collecting a section of the beam at the detector in such a mannerthat the various components are combined in the measured signal, theeffect of interference within the wafer or optical elements can begreatly reduced. The use of multiple laser sources that are placed closetogether so that they can be combined to form one optical beam wouldalso achieve similar results. Large area emitting sources thatincorporate multiple laser elements could be useful in this embodiment.

In still another embodiment, the use of multiple coherent light sourcescould help reduce the temporal coherence of the beam by introducing arange of wavelengths. For instance, some types of diode lasers do nothave a tightly controlled emission wavelength. The natural scatter indevice characteristics may result in a broader effective spectrum forillumination. If desired, light devices could deliberately be selectedto provide a greater range of wavelengths. Also, some light sources withthe same nominal specification could be deliberately run at differenttemperatures to create a range of wavelengths, since in many cases theoutput wavelength is sensitive to the device temperature.

In another embodiment, interference effects may be reduced by using thecombination of an incoherent light source and a coherent light source.In this embodiment, the incoherent light source can be used as thecalibrating light source at lower temperatures. At lower temperatures,transmission measurements are not as dependent upon various parameterssuch as the refractive index and the thickness of the substrate.Relatively lower temperatures can include temperatures less than about700° C., and particularly less than about 500° C.

At relatively high temperatures, such as greater than about 500° C., andparticularly greater than about 700° C., the calibrating light sourcecan be a coherent light source. At higher temperatures, coherent lightsources may be preferred because they provide greater power at aparticular wave length and better wavelength definition. The same ordifferent detectors can be used to detect transmission from thecalibrating light sources. In this embodiment, for instance,transmission measurements can be taken at lower temperatures using anincoherent light source. These transmission measurements can then beused to determine the temperature of the wafer at higher temperatures inconjunction with transmission measurements taken using the coherentlight source.

Regardless of whether a coherent or incoherent light source is used, itis desirable that the power supply to the light source be configured inorder to provide a stable and repeatable intensity for mostapplications. Although the technique of the present invention isself-calibrating, in a sense that the transmission measurements when thewafer is hot are made relative to a transmission value obtained when thewafer is at temperature where its transmission is known, there is stilla possibility of some drift in the light source intensity while thewafer is heated through the calibration recipe. This variation can bereduced by ensuring that the power supply to the light source is stable.Optionally, an optical detector can be used to sample some of the lightemitted from the calibrating light source and hence generating a signalfor either monitoring the light source or controlling its intensity sothat it is stable and repeatable during the measurements.

Calibration Wavelength

The operating wavelength for the transmission measurement should beselected in order to provide good sensitivity to temperature. Ingeneral, the wafer should be sufficiently transparent to allow anaccurate measurement of the transmitted infra-red signal intensity. Inmost situations, this requires the transmissivity at the probewavelength to be greater than 10⁻⁶.

The wafer transmission should also be sensitive to the wafertemperature. For example, it is desirable that, near the specifiedcalibration temperature, the wafer transmission changes by greater thanabout 0.5% of its value for a 1° C. change. Ideally, the transmissionchanges by about greater than 5% for a 1° C. temperature change.

In some applications, it can be advantageous for the calibration waferto be close to opaque at the pyrometer wavelength while it stilltransmits a measurable amount of radiation at the transmission testwavelength. This may be desirable so that the calibration reflects atypical condition for the optical properties of a standard thicknesswafer at the calibration temperature of interest. In practice, it wouldbe sufficient for the wafer to have a transmissivity of less than about0.01 at the pyrometer wavelength, while the transmissivity at theinfra-red transmission probe wavelength is greater than about 10⁻⁶. Thiscondition can often be met because the absorption coefficient of silicontypically exhibits a minimum value in the near infra-red range, so thatthe transmission measurement can be performed in this wavelength range,under conditions where the wafer is opaque at other wavelengths. Thisconcept may be especially useful for calibration at temperatures greaterthan about 800° C., where standard thickness wafers usually are highlyopaque at optical and infrared wavelengths.

In other applications, it may be desirable to know the wavelength of themeasurement to an accuracy better than that of the nominal value givenfor an emitting source or optical filter. In this case, the spectrum ofthe emitting source or the transmission of the optical filters can bemeasured so that the exact measurement wavelength is established. Thisvalue can then be used to improve the accuracy or repeatability ofreadings. For example, if the measurement system is used in multipleprocessing systems, it may be important to correct for the effect ofvariations in the measurement wavelengths between systems, in order toachieve the most consistent calibrations on different systems. One wayto do this is to use an algorithm that takes account of the wavelengthinput and wavelength variations when converting a transmitted lightsignal into a deduced temperature.

In one embodiment, as stated above, the system of the present inventioncan be configured so that temperature measurements are performed over arange of wavelengths instead of a single wavelength. Measuring thetemperature over several wavelengths can provide various advantages. Forinstance, a measurement of a transmission spectrum allows the wafertemperature to be determined without having to perform the normalizationstep of measuring the transmitted intensity at a temperature where thewafer transmission is known, such as when the wafer is cool. It ispossible to determine the temperature because the shape of theabsorption spectrum can be identified and used to determine the wafertemperature. In this manner, it is not necessary to take a temperaturereading where the transmission of the wafer is not a strong function oftemperature.

More particularly, by measuring the transmission for at least twowavelengths, a relative transmission at those wavelengths can bedefined. The broader spectrum (i.e., multi-wavelength) can be collectedand matched to a model, using temperature as a parameter for the fit. Inthis embodiment, there may be advantages and better resilience againstfluctuations in wafer thickness or unexpected variations in otherparameters such as coatings, etc.

These circumstances can also be useful if trying to calibrate apyrometer for operation at very low temperature, such as less than about200° C. In these circumstances, the normalization temperature typicallyhas to be very close to room temperature, to prevent the transmissionfrom being sensitive to the exact value of the normalizationtemperature. This is especially helpful in applications where thethermal processing environment is already warm when the wafer is added,thus making it very difficult to take a temperature reading below about30° C.

Spectral measurements taken over a range of wavelengths also tend tomake the temperature measurement less sensitive to drift in the lightsource intensity or detector characteristics, optical contamination andnoise. Spectral measurements can be taken using an incoherent lightsource that emits light at different wavelengths or can be done by usingmultiple light sources, such as multiple incoherent sources or multiplecoherent sources. The measurement can be taken at different wavelengthsat the same time or at different times, such as consecutively.

As shown in FIG. 1, the calibrating light source 23 is positioneddirectly above the light detector 42. The light that is emitted from thecalibrating light source 23 can be configured to shine directly onto thewafer or, alternatively, can be incident on the wafer at a non-normalangle of incidence. In some applications, it may be desirable for thelight to contact the wafer at an angle in order to reduce reflectionfrom the wafer surface. For example, if the light is in theP-polarization plane and is incident at an angle near the criticalangle, then the reflectivity of the surface approaches zero. By reducingreflectivity, not only is a stronger signal transmitted through thewafer, but interference effects within the wafer are reduced. In oneembodiment, it may be desirable to use polarized light at a non-normalangle of incidence.

Reducing the Effects of Stray Radiation

As described above, in order to improve accuracy, the measurement andsystem of the present invention should have the ability to distinguishbetween the transmitted light signal and signals that may arise fromstray radiation sources. In the embodiment illustrated in FIG. 1, thewafer 14 is only heated from one side. In this case, the wafer itselfacts as a screen to decrease the possibility of stray light reaching thelight detector 42, since the detector is located on a side of the waferopposite the light sources 24. There are various other mannersavailable, however, to decrease stray light and interference especiallyfor use in systems that contain light sources on both sides of thewafer.

In general, for most applications, the calibrating light source 23should be modulated so that the light signal received by the lightdetector 42 is an ac signal or other similar signal, that can bedistinguished from signals that arise from stray light originating fromthe wafer, the lamps or other radiation sources. The transmitted lightsignal can be extracted from the detected signal in order to removestray light by a variety of techniques including spectral filtering,signal averaging and phase-sensitive detection, for example, with alock-in amplifier. The above approaches can extract a very small acsignal even in cases where the signals are contaminated with largeamounts of noise.

In one embodiment, the illumination optics can also be configured tomaximize the amount of transmitted light that reaches the light detector42. For instance, as shown in FIG. 5, the beam of light emitted by thecalibrating light source 23 can be collimated. Another approach involvesusing a light source having a filament temperature which issignificantly hotter than the wafer, such as by using a tungstenfilament, so that the light source is much brighter than the wafer atthe wavelength being monitored. Coherent light sources can also be usedfor this purpose. The use of laser sources may also permit the deliveryof significantly higher optical powers through the wafer and into thelight detector 42.

Besides using illumination optics to strengthen the transmission,detection optics can also be configured to minimize stray light. Forinstance, detection optics can be used that only view the region of thewafer that is illuminated. The detection optics can have angularacceptance characteristics that favor collection of light that emanatesfrom the light source, rather than the wafer. For example, theblack-body radiation emitted by the wafer tends to radiate in alldirections from the wafer surfaces. By restricting the angularacceptance characteristics of detection optics so that they tend toaccept light traveling in the direction defined by the light sourceoptics, the ratio of emitted radiation to transmitted light can bedecreased. Similar considerations can be applied to the radiation ofstray light from the heating lamps 24. Heating lamps 24 can createradiation that is transmitted through the wafer or reflected from thebackside of the wafer depending upon where the lamps are located. If thesystem includes detection optics that do not accept direct light pathsfrom the lamps through the wafer or light paths corresponding to thecondition for specular reflection of light off the wafer, then the straylight from the lamps can be greatly reduced.

Besides illumination optics and detection optics, the system of thepresent invention can also include an optical filter that rejectsradiation from outside a range of wavelengths around the wavelengthbeing used for the transmission measurement. For instance, thiswavelength range can be defined by an interference filter.

Especially when using an incoherent light source as the calibratinglight source 23, such as a tungsten-halogen lamp, it is desirable forthe filter to have good blocking characteristics outside the desiredpass band. Specifically, the incoherent light source may emit light thatis modulated at all wavelengths, and as a result the light detector 42can receive an AC signal that arose from modulation at wavelengths otherthan the filter pass band. For instance, in most applications, thefilter transmission should be below about 10⁻³ outside the pass band,and when the measurements are performed using wavelengths less thanabout 1.4 microns, desirably the filter has the above blockingcharacteristics, especially on the long-wavelength side of the passband, because the silicon transmission is higher at longer wavelengths.

In one embodiment, the filter blocking can be below about 10⁻⁶. Thistype of blocking can be achieved by combining two interference filtersin such a manner that their combined blocking reaches the desired value.In some cases, it may be convenient to put one of the filters on theoutput of the lamp source to restrict the wavelength range that isemitted from the lamp.

If the calibrating light source 23, however, is a coherent light sourcesuch as a laser, filters are not needed, because the range of modulatedwavelengths is inherently small. Nevertheless, in one embodiment, afilter can be used to decrease the amount of stray radiation from thewafer and the heating lamps.

Even when using an incoherent light source, a filter may not be desired.Alternatively, a multiple band filter can be used. Operating without anarrow band filter or with a multiple band filter can provide variousadvantages in some circumstances. For instance, when operating without afilter, one detector can be used to detect radiation from severaldifferent light sources emitting radiation at different wavelengths,without the need for switching filters.

In still another embodiment of the present invention, the calibratinglight source 23 can emit polarized light. In this embodiment, the systemcan include detection optics that select a specified polarization, forremoving stray light. In order to produce polarized light, thecalibrating light source 23 can be a laser that is inherently polarized.Alternatively, the calibrating light source 23 can include a polarizer.

Calibration Wafer

The calibration wafer used in the system of the present invention isselected so that light transmission through the wafer is a function oftemperature at the wavelength observed by the light detector 42. Thewafer can be optimized for the purposes of calibration in any number ofways including varying the thickness, doping the wafer, applying surfacecoatings to the wafer, and modifying the surface texture.

In one embodiment, the calibration wafer can be a plain silicon wafer.The wafer can be a lightly-doped silicon wafer with a resistivitygreater than about 0.5 Ωcm. In this embodiment, standard thicknesswafers can be used. Currently, standard wafer thicknesses are 725microns for a 200 mm wafer and 775 microns for a 300 mm wafer. For mostapplications, the wafer is polished on both sides in order to improvethe repeatability of the calibration standard.

The calculation of the temperature of the calibration wafer depends uponthe wavelength of light being transmitted through the wafer and thethickness of the wafer. In this regard, the thickness of the wafershould be known with some accuracy. In one embodiment, it may bedesirable to correct for variations in the wafer thicknesses that areused during temperature measurements. For example, the wafer thicknesscan be measured and this data can be entered into the algorithm used todetermine the wafer temperature. Alternatively, the system of thepresent invention can be equipped with an instrument that measures thewafer thickness and automatically provides this information to, forinstance, the controller 50 for use during temperature measurements. Thewafer thickness instrument can measure the thickness of the wafer insidethe thermal processing chamber or can measure the thickness of the waferprior to inserting the wafer into the chamber. Such an instrument,however, is not necessary if the calibration wafer is made so thatthickness variations are negligible.

In order to inhibit reflection from the wafer of light emitted from thecalibrating light source 23, in one embodiment, the calibration wafer 14can be coated with an anti-reflection coating. Coating the wafer with ananti-reflection coating not only reduces the amount of light that isreflected from the wafer but also reduces the effect of interferencethat occurs inside the wafer.

Referring to FIGS. 6 and 7, two embodiments of a calibration wafer 14containing anti-reflective coatings are shown. In FIG. 6, a singleanti-reflective coating 70 is present on the top surface of thecalibration wafer 14. In FIG. 7, on the other hand, an anti-reflectivecoating 70 is placed on the top of the wafer 14, while a secondanti-reflective coating 72 is placed on the bottom of the wafer.

When anti-reflective coatings are applied to the wafer, reflection oflight at the infra-red wavelength of interest can be significantlyreduced and interference effects are suppressed. The anti-reflectivecoating(s) also provides a benefit in that it increases the amount oflight transmitted by the wafer, improving the accuracy of thetransmission measurements.

Anti-reflective coatings, however, may affect the emissivity of thewafer at the pyrometer wavelength of the pyrometer that is beingcalibrated. For calibrations that are performed at high temperatures,where the wafer is usually opaque at the pyrometer wavelength, it may bedesirable to only coat one surface of the wafer as shown in FIG. 6.Specifically, the surface of the wafer facing the pyrometer can remainuncoated so that the desired effect is achieved without influencing theemissivity of the pyrometer wavelength.

In another embodiment of the present invention, as shown in FIG. 8, thecalibration wafer 14 can include not only an anti-reflective coating 70but a coating 74 having a specified emissivity. In particular, in manysituations the temperature measurement device contained within thethermal processing chamber is calibrated with a set of wafers that havedifferent spectral emissivities at the pyrometer wavelength. Wafershaving different emissivities are used in order to correct foremissivity effects in pyrometry and make the temperature measurementsindependent of the spectral emissivity of the target wafer.Consequently, in some embodiments, different calibration wafers can beused according to the present invention having particular emissivities.

In general, any suitable anti-reflective coating can be used accordingto the present invention. Such coatings can be made from, for instance,silicon dioxide, silicon nitride or a combination of the two. Suchmaterials can also be used to form coatings having a particularemissivity. Silicon films can also be used for this purpose. Siliconfilms have a large refractive index which may be useful in someapplications. The coatings 70 or 74 as shown in FIG. 8 can also be madefrom a dielectric film. The dielectric film can be a multi-layered filmspecially designed to have the appropriate reflectivity and/oremissivity at the desired wavelength. Such films are known in the artand can be obtained from Deposition Sciences, Inc. of Santa Rosa, Calif.

When present, the coating 70 or 74 can affect the transmission of theinfra-red light being emitted by the calibrating light source 23. Thesevariations can be taken into account, either through calculations ormeasurements, and incorporated into the algorithm that is used tocalculate the temperature of the calibration wafer. It is also possibleto form different coating designs that act as anti-reflective coatingsat the infra-red transmission wavelengths, and act as reflective oranti-reflective coatings at the pyrometer wavelength.

Besides possibly improving the accuracy of temperature measurements madeaccording to the present invention, the use of anti-reflective coatingsand other types of coatings may provide other benefits. For instance, bychoosing the optical properties of the coatings on the calibrationwafer, it is possible to probe various aspects of the temperaturemeasurement system's performance, and provide diagnostics of the natureof problems. For example, a test which checks whether the pyrometeroptics and electronics are operating correctly can use a wafer backsidecoating which is “black” at the pyrometer wavelengths, so that thesignal at the pyrometer is not affected by other aspects of the systemoptics, such as affects arising from multiple reflections between thewafer and the chamber walls. This is especially useful if the systemuses reflective enhancement of the wafer emissivity to assist withemissivity-independence. In these systems, the reflectivity of theauxiliary reflector used for emissivity enhancement plays a key role inthe system performance. A test which can reveal if the reflective platereflectivity has degraded, uses a coating which is highly reflective atthe pyrometer wavelength, so that multiple reflections from thereflective plate become involved. This kind of wafer can also be usefulfor checking that emissivity correction systems are working in any kindof system. These type of tests can distinguish various types of problemswithout requiring the chamber to be opened for inspection.

Coatings as described above can also help reduce the effect ofprogressive thermal degradation on the wafer's optical propertiesbecause of oxidation and surface roughening. Consequently, multiplethermal cycles can be applied to the calibration wafer withoutdegradation. For example, a thin layer of oxide, such as less than about30 nanometers thick, can protect the silicon surface from thermaletching that results from active oxidation by low concentrations ofoxygen and water vapor in the ambient gas. A thin film of nitride canserve a similar purpose. The films can be included as the top layers inthe coating stacks used for anti-reflective coatings or for altering thespectral emissivity as described above. Further, by choosing films ofoxide that are less than about 30 nanometers thick, the impact on theemissivity of the wafer by the film can be minimized.

Besides using silicon wafers as the calibration wafers, it should alsobe understood that various other types of wafers can be used. Ingeneral, the calibration wafer can be made from any suitable materialthat has a temperature dependent transmission or reflection spectrum.For instance, the calibration wafer can also be made from germanium,silicon carbide, gallium arsenide, aluminum arsenide, indium phosphide,gallium phosphide, gallium nitride and their alloys. In fact, some ofthe above materials may be useful to extend the temperature range fortransmission.

When using silicon as the calibration wafer, silicon is known to havegood temperature transmission properties at temperatures less than about850° C. for standard thickness wafers. As described above, however, attemperatures greater than 850° C., silicon begins to absorb more lightenergy making temperature measurements more difficult. In oneembodiment, in order to extend temperature measurements according to thepresent invention to higher temperatures, thinner wafers can be used.For example, the predicted internal transmission at a wavelength of 1.5microns for a 250 micron wafer at 950° C. is 0.012, which is more than300,000 times higher than the transmission for a wafer having aconventional thickness of 725 microns. By using very thin wafers, the IRtransmission can be extended to higher temperatures. Further, using thinwafers will not have an impact on temperature measurements taken by thepyrometer, so long as the wafer remains opaque to radiation at thewavelength of the pyrometer.

Various practical problems, however, can arise when using relativelythin wafers. For instance, the wafers may sag under their own weight athigh temperatures and may even undergo plastic deformation. FIGS. 9through 18 are examples of various embodiments of calibration wafershaving thin areas where these types of problems can be minimized.

For example, referring to FIG. 9, a calibration wafer 14 is shown havinga thin area for transmission measurements. In this embodiment, the wafer14 includes a hole or passage 80 at a location that is in the field ofview or near the field of view of the light detector 42 and thetemperature measurement device or pyrometer 27 that is to be calibrated.In this embodiment, a silicon member 82 having the desired thickness isplaced on top of the hole 80. The thickness of the silicon member 82 canbe, for instance, less than about 300 microns, particularly less thanabout 200 microns, and more particularly less than about 150 microns. Byplacing the silicon member 82 over the hole 80 formed into the wafer 14,a calibration wafer can be created that is relatively strong for use inthermal processing chambers, but yet has a thin section for use inmaking measurements according to the present invention.

The silicon member 82 can be positioned in place over the hole 80 andsecured to the wafer using various methods. For example, in oneembodiment, the silicon member 82 can be bonded in position. Anysuitable bonding method can be used such as anodic or thermal bonding.

In an alternative embodiment, a retaining cover can be placed over thewafer in order to hold the silicon member in place. The retaining membercan be made from silicon, silicon carbide, silica (quartz) or sapphire.If the retaining cover is not transparent, it may be necessary in someapplications to place a hole or passage through the retaining cover thatis in alignment with the silicon member in order to take accuratetransmission measurements. When using a transparent material, such asquartz or sapphire, however, then such an opening will not be needed.

In one embodiment, the silicon member 82 can have different thicknessesto cover different temperature ranges. In this embodiment, the varyingthickness range may allow the system to use fewer infra-red transmissionwavelengths to calibrate a wider temperature range. Further, if multiplepyrometers are to be calibrated, the calibration wafer 14 can includemany different holes at the various different locations as shown in FIG.13. Alternatively, a set of different calibration wafers can be usedthat each contain a single or a few holes at selected locations.

As shown in FIG. 10, in one embodiment, the calibration wafer 14 caninclude a recess 84 that contains a ledge for placement of the thinmember 82. In this embodiment, the thin member 82 remains in alignmentduring processing and is easier to position on top of the wafer.

In another embodiment, instead of forming a hole in the calibrationwafer 14 and then covering the hole with a thin member, a thin area onthe wafer can be formed by forming a recess 86 in the wafer as shown inFIGS. 11 and 12. As shown, recess 86 forms a thin section 88 in thewafer 14. The recesses 86 can be formed by any suitable manner, such asby machining or etching. As shown in FIG. 14, in one embodiment, therecess 86 has a graded thickness forming a frustoconical shape. Further,the corners can be curved as well. The graded thickness and curvedcorners reduce thermal gradients at the edges of the thin sectionreducing the chance of mechanical failure and reducing stress.

When the calibration wafer includes thin sections, there is apossibility of temperature gradients developing in the wafer duringheating. In order to reduce the effect of differences in thermalradiative properties between the thin section and the remainder of thewafer, the wafer can include a coating. The coating can be, forinstance, a single layer or multi-layer coating. The coating can be madefrom materials such as silicon dioxide, silicon, polysilicon, and/orsilicon nitride. The coating can be designed in order to better matchthe emissivity and absorptivity of the thin and thick sections. In oneembodiment, the coating can be made from any of the anti-reflectivecoatings described above.

In an alternative embodiment, the calibration wafer can include a fillmember that is placed adjacent to the thin section for reducing thethermal mass differences. The fill member can be made from, forinstance, quartz or aluminum oxide, such as sapphire.

Further embodiments of a calibration wafer 14 made in accordance withthe present invention are shown in FIGS. 16, 17 and 18. In theseembodiments, the calibration wafer 14 includes a light transmissionregion 90 that is defined by a plurality of channels 92. Thus, thetransmission regions 90 include thin regions mixed with thick regions,wherein the thick regions have the same thickness as the wafer itself.The channels 92 can only extend through a portion of the thickness ofthe wafer as shown in FIG. 16 or can pass through the entire thicknessof the wafer as shown in FIGS. 17 and 18. In FIGS. 17 and 18, the thickareas 94 are attached to a thin member 82. Further, in FIG. 18, the thinmember 82 is placed within a recess 84 formed into the wafer 14. In theembodiments illustrated in FIGS. 17 and 18, the thin member 82 can beattached, e.g. by bonding, to the wafer 14.

In the embodiments shown in FIGS. 16, 17 and 18, when the calibrationwafer is heated to relatively high temperatures, light transmission isprimarily determined by the channels 92. Therefore, the presence of thethick members 94 does not interfere with the temperature measurements.This embodiment of the calibration wafer provides several advantages.For instance, by keeping the thin region small and mixed with the thickregions, problems of lateral temperature gradients and thermal stressescan be reduced. Further, it may be easier to align a pyrometer, acalibrating light source, or a light detector with the transmission area90.

When incorporating the embodiments illustrated in FIGS. 16 through 18,it may be desirable in some applications to know the ratio between thethick areas and the thin areas in order to correct for the presence ofthe thick areas during measurements. This ratio can be measured in anysuitable manner. For instance, the ratio can be determined usingphysical measurements or optical measurements can compare the fluxtransmitted with the wafer in a beam path compared to that without thewafer present. The latter measurement can be performed at any convenientwavelength where the thick region is effectively opaque. The areas couldalso be established with high accuracy a priori, for example by formingthem using optical lithography or other similar technique. The methodsof micro machining could be employed to create suitable structures.Determining the thickness of the thin regions can be accomplished by,for instance, performing a transmission measurement at a knowntemperature at a suitable wavelength.

When forming the transmission area 90, it should be understood that thewafer can contain multiple areas as shown in FIG. 14 or can contain asingle transmission area that covers the entire wafer as shown in FIG.15.

When incorporating the calibration wafer illustrated in FIG. 17 or 18,in one embodiment, it may be preferable to orient the wafer such thatthe thin member 82 is oriented upside down so that the thin member facesthe pyrometer being calibrated. This configuration can be advantageousbecause the pyrometer would view an opaque homogeneous surface, ratherthan an array of holes.

Referring to FIG. 19, another embodiment of a calibration wafer 14 isillustrated. In this embodiment, the calibration wafer 14 includes athin silicon layer 100 placed on top of a transparent substrate 102. Forinstance, the transparent substrate can be aluminum oxynitride, spinel,fused silica or sapphire. The result is a strong calibration wafer thathas the performance of a very thin silicon material.

The silicon layer 100 can be placed on the transparent material in anumber of different ways. For instance, the silicon layer can be bondedto the transparent material or, alternatively, the silicon layer can beformed by deposition on top of the transparent material.

Referring to FIG. 20, another embodiment of a calibration wafer 14 isshown. In this embodiment, the calibration wafer 14 includes a thinsilicon layer 100 placed on top of a silicon substrate 106. The siliconlayer 100 is isolated from the silicon substrate 106 by an insulatorlayer 104. Insulator layer 104 can be made from an oxide, such assilicon dioxide. As shown, the calibration wafer 14 further includes atransmission area 90 containing channels 92 formed into the siliconsubstrate 106.

Referring to FIG. 21, a similar embodiment of a calibration wafer 14 tothat shown in FIG. 20 is presented. In FIG. 21, however, the channels 92are formed through the oxide layer 104.

In the embodiments illustrated in FIGS. 16, 17, 18, 20 and 21, a siliconwafer acts as a support for the transmission areas 90. Instead of beingmade from silicon, however, the support substrate can be made from othermaterials. For example, in one embodiment, the substrate can be madefrom silicon carbide which would give advantages in terms of strength,high thermal conductivity, chemical durability and mechanicaldurability.

In another embodiment of the present invention, the calibration wafercan include opaque areas made from a substantially opaque material andtransmissive areas made from, for instance, silicon. This type ofcalibration wafer may be well suited for use in low temperaturecalibrations. In this embodiment, the pyrometer being calibrated can bepositioned to sense thermal radiation being emitted by the wafer in theopaque areas. The transmission measurements, however, can be taken inthe transmissive areas.

Substantially opaque materials that can be used to construct the waferinclude silicon doped with a material, such as boron, arsenic, orphosphorous. Other examples of substantially opaque materials includemetal films such as titanium films, cobalt films, nickel films, andtungsten films, and films made from metal silicides such as titaniumsilicide, cobalt silicide, nickel silicide and tungsten silicide. Someother conducting materials, such as titanium nitride could also be used.

As stated above, in some applications, it may be desirable to know theratio between the thick and the thin areas of transmission areas 90. Inone embodiment, however, this information may not be needed as long asthe thick areas are opaque at the low temperature used for normalizingthe signal in the infra-red transmission method. This configurationwould eliminate the need for separate measurement, since the methodwould be self-correcting. Opacity could be measured by several methods.For instance, in one embodiment, normalization of the signal can occurat a temperature where it is known that the thick sections are opaquebut the thin ones are completely transparent. Alternatively, a coatingcan be applied to the thick areas, where the coating is stronglyabsorbing or strongly reflecting at the chosen probe wavelength. Anothertechnique would be to heavily dope the thick regions, for example byion-implantation.

In one embodiment, the calibration wafer can be made predominantly froma material that is opaque at the normalization temperature. The wafercan then include a thin coating of silicon, similar to the wafer shownin FIG. 19. In this embodiment, the support wafer can be made from anysuitable material that is opaque at the wavelength of interest, such asbeing made from a heavily doped material.

In another embodiment of the present invention, different calibrationwafers can be used that are made from different materials. Usingdifferent materials may allow more measurements to be taken at a widerrange of temperatures. The other materials that can be used includesilicon carbide, aluminum nitride, gallium nitride or gallium phosphide.In particular, a material for construction of the calibration wafer canbe chosen that is transparent for the IR transmission wavelength yet isopaque at the pyrometer wavelength.

For purposes of illustration, below is a table which lists somepractical temperature ranges at three proposed wavelengths and threeproposed wafer thicknesses. In this embodiment, the “low T” headingrepresents a practical lower temperature within the range for any givenwavelength/thickness combination and the “high T” heading represents theupper temperature limit for the same combination. The lower temperaturelimit, in this embodiment, was set by diminishing temperaturesensitivity. In particular, the lower limit represents the temperatureat which the transmitted light signal will no longer vary by at least0.1% of its value per 1° C. change (represented as sens(%/° C.)). Theupper limit, on the other hand, was set at a point where the waferbecame very opaque. In this embodiment, the high temperature range wasat a point where the wafer no longer transmitted light energy at leastat a rate of 10⁻⁸ transmission (1 part in 100,000,000). In the table,this is indicated as “int trans”. The wavelengths 1310 nm and 1550 nmwere chosen since they are convenient for commercial light sources. Thewavelength of 1200 nm was an arbitrary choice.

Wafer Wavelength (nm) Thickness 1200 1310 1550 (Microns) Low T High TLow T High T Low T High T 725 T (° C.) 125 600 360 745 415 885 int trans0.96 9.283E−09 0.943 1.08E−08 0.95 1.29E−08 sens (%/° C.) 0.1 14 0.115.6 0.1 15.9 100 T (° C.) 250 975 425 1060 560 1160 int trans 0.931.192E−08 0.95 1.53E−08 0.93 1.37E−08 sens (%/° C.) 0.1 7.9 0.1 9.1 0.110.8 60 T (° C.) 300 1085 460 1165 600 1250 int trans 0.92 1.536E−080.94 1.15E−08 0.93 1.53E−08 sens (%/° C.) 0.1 7 0.1 8 0.1 9.5Calibration Procedures

Different methods for carrying out the process of the present inventionwill now be discussed. In one embodiment, for instance, a selectedcalibration wafer is loaded into the thermal processing chamber. Acalibration recipe is then run. The recipe causes the wafer to be heatedthrough a predetermined temperature-timed cycle while the data requiredis acquired. The data can include pyrometer readings and infra-redtransmission data. An algorithm as described above interprets thetransmission data in terms of wafer temperature, and calculatesparameters that are used to correct the pyrometer system so that thetemperatures deduced from the pyrometer system match those deduced fromthe infra-red transmission system. Of course, besides pyrometers, thesystem can also be used to calibrate other temperature measurementdevices.

A typical recipe can include one heating cycle or multiple heatingcycles, where the wafer is ramped to a given temperature, as determinedby the pyrometer system, and kept there for a fixed period of time whiledata is acquired before being ramped to a second temperature and so on.The recipe covers the temperature range that is to be calibrated. Therecipe can include a portion at a temperature where the transmission ofthe wafer is known, and where it is not a strong function oftemperature. This portion of the recipe allows the infra-red system toacquire the transmission signal that is used to normalize the signals,so that the effects or variations in the optics of the infra-redtransmission measurement system can be eliminated.

If a coherent light source is used, this portion of the recipe may alsoinclude a slow temperature ramp or oscillation that introducessignificant changes in the optical path length through the wafer. Thesevariations alter the conditions for optical interference between thebeams of light deflected from the top and bottom surfaces of the waferand hence result in an oscillating transmission signal. The datarecording system can collect this transmission data and then averageover time so that the effect of the oscillation is reduced. Thisapproach may be useful for stages in the recipe that are performed underconditions where the wafer transmissivity is high enough for multiplereflections within the wafer to be a factor.

An alternative type of recipe is to collect data dynamically through aramp-up and/or ramp-down recipe. In this case, the ramp rate would haveto be sufficiently slow for there to be negligible time lag between theoutput of the two measurement systems, and also for adequate signalaveraging to occur to reduce experimental noise. In this embodiment,when using a wafer having thin sections, the thermal response of thethin sections in relation to the thick sections should be similar.

It may also be useful in some applications to perform a measurementusing the transmission system before the calibration wafer is loadedinto the chamber. The ratio of the signal with the wafer in place ascompared to that without a wafer provides an estimate of thetransmissivity of the wafer. The measured transmissivity of the wafercan be useful, for example, to check that the correct wafer was loaded,or to test for degradation of the wafer. The transmission signal levelwhen there is no wafer present is also a useful indicator of the statusof the optical system for transmission measurements.

In one alternative embodiment to the above procedure, more than onecalibration wafer can be used, each of which can be automatically loadedinto the processing system. The multiple calibration wafers can serveseveral purposes. For example, different wafers with differentcharacteristics can be loaded into the thermal processing chamber toallow adequate performance in a high-temperature range as well as alow-temperature range. For example, wafers with thin areas may be usedfor higher temperatures. Also, the wafers can include different coatingsto allow the necessary calibrations for the pyrometer system to giveemissivity-independent temperature measurements.

In one application of the system of the present invention, it may bedesirable to calibrate the pyrometer system at just one or at a fewselected temperatures, in order to rapidly verify that the system isworking properly. In this case, the calibration wafer can be loaded anda very simple recipe performed, for example ramping to one testtemperature and collecting pyrometer and infra-red transmission data.This procedure can be performed routinely at user specified intervals.The data required for this procedure can be collected in order to trackany drift in the pyrometer system's characteristics. Once a noticeablediscrepancy is observed between the temperature measurement devices andthe determined transmission temperature, the system can be recalibrated,or maintenance or chamber cleaning can be performed.

In some applications, the transmission measurement system of the presentinvention can also be operated during normal processing of wafers.Specifically, the system of the present invention can be used duringnormal processing of wafers where the wafer substrate doping is knownand the wafer transmits a measurable amount of infra-red radiation. Forexample, the process recipe can be run and while the pyrometer is takingreadings of the temperature, an algorithm checks the readings from thepyrometer against those of the IR transmission. If the differencebetween the readings exceeds some specified limit, that may be anindication that corrective action is needed. This method can be carriedout over part or all of the specified temperature range or recipe time.

In fact, if enough information is known about the wafer being processed,the temperature measurement device in the processing chamber can becalibrated while the actual wafer is being processed. The calibrationcan, for example, happen in the early part of the temperature cycle,where the wafer is still at a temperature low enough for the IRtransmission system to function adequately. In this application, thetemperature deduced from the IR transmission can serve to improve theemissivity estimate of the wafer and hence improve the pyrometer'saccuracy.

In one embodiment, the transmission measurement system of the presentinvention can be used to monitor process uniformity. In this embodiment,the system preferably includes multiple transmission measurement devicesthat can serve to measure light transmission at several locations on thewafer. During normal processing of wafers, or a test run using a specialwafer, the transmission measurement system can monitor the temperatureof the wafer at the multiple locations. This information can then beused to check and determine that the heating cycle programmed into thesystem is operating correctly at all the locations where a transmissionmeasurement is performed. More particularly, in this embodiment, thetransmission measurement system provides a real-time method for ensuringthat the heating cycles are set properly. For instance, the transmissionmeasurement system can determine the temperature profile of a heatingcycle including ramp-up rates and ramp-down rates for accuracy. Bymaintaining the heating cycles at all the monitored locations withinpreset limits, process uniformity is improved, whether the process beingcarried out in the chamber is annealing or depositing materials on awafer.

In principle, the heating system of the thermal processing chamber canbe controlled using feedback from the temperature deduced by theinfra-red transmission measurement of the present invention in aclosed-loop mode. This might be useful in the case where the pyrometersystem has never been calibrated before and as a result its readings aretoo inaccurate to allow for it to be used to control the heating cycle.

Besides being used in a closed-loop control cycle, the infra-redtransmission measurement system of the present invention can also beused in an open-loop control cycle. In an open-loop control cycle, nofeedback signal is used to control the light sources. Instead, the lightsources are preprogrammed to run a particular heating cycle.

Use of the transmission measurement system of the present invention todetermine the temperature of wafers independent of any other temperaturemeasurement devices is particularly useful at temperatures less thanabout 800° C., where most wafers transmit a measurable amount of IRradiation. In this embodiment, the transmission system of the presentinvention can be used to monitor the temperature of the wafer beingprocessed at lower temperatures, while the normal temperaturemeasurement system can be used to monitor the temperature of the waferat higher temperatures. For instance, the transmission measurementsystem, in this embodiment, is particularly well-suited for use inconjunction with pyrometers that are not as accurate at lowertemperatures.

Ultimately, a full closed-loop control can be applied throughout theentire process cycle, from room temperature upwards, eliminating theneed for an open-loop heating block that is normally used to heat awafer up to the temperature where pyrometer control takes place. Thisembodiment can improve the overall process control and throughput, whilereducing the effort needed to create recipes. Further, in applicationswhere the peak process temperatures are less than about 800° C., theentire heating cycle could be controlled by the transmission measurementsystem of the present invention.

The presence of an in-situ transmission measurement system can alsoprovide various other useful information during the processing ofwafers. For instance, the system of the present invention is alsoconfigured to provide information as to whether or not the wafer beingprocessed is opaque at the temperature or temperatures of interest. Thisinformation can be used to automatically select an optimal temperaturemeasurement and control algorithms to handle, lightly-doped,heavily-doped and metalized wafers in optimal ways which account fortheir very different power absorption characteristics.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1. A calibration wafer intended to be used for calibrating temperaturemeasurement devices in thermal processing chambers, the calibrationwafer comprising: a wafer having a diameter of at least 200 mm, thewafer comprising a first region and a second region, the thickness ofthe wafer at the first region being less than about 150 microns and, thefirst region having a temperature dependent optical transmissioncharacteristic so that the wafer can be use for calibrating temperaturemeasurement devices; wherein the thickness of the wafer at the firstregion is less than the thickness of the wafer at the second region;wherein at a calibration temperature, the first region is transparent toinfrared radiation and the second region is opaque to infraredradiation.
 2. A calibration wafer as defined in claim 1, wherein thethickness of the wafer at the first region is less than about 100microns.
 3. A calibration wafer as defined in claim 1, wherein the waferincludes at least two first regions.
 4. A calibration wafer as definedin claim 1, wherein the first region comprises silicon.
 5. A calibrationwafer as defined in claim 1, wherein the wafer is made from silicon. 6.A calibration wafer as defined in claim 1, wherein the wafer includes acoating on at least one side of the wafer.
 7. A calibration wafer asdefined in claim 1, wherein the first region comprises a thin piece ofmaterial placed over a hole defined by the wafer.
 8. A calibration waferas defined in claim 1, wherein the first region is surrounded by walls,the walls having an incline surface.
 9. A calibration wafer as definedin claim 1, wherein wafer includes a plurality of first regions groupedtogether.
 10. The calibration wafer as defined in claim 1, wherein thecalibration temperature is greater than or equal to about 850° C. 11.The calibration wafer as defined in claim 1, wherein at the calibrationtemperature, the first region is transparent to radiation having awavelength in the range of about 1.0 micron to about 2.0 microns and thesecond region is opaque to radiation having a wavelength in the range ofabout 1.0 micron to about 2.0 microns.
 12. A calibration wafer asdefined in claim 7, wherein the wafer includes multiple locations wherethe plurality of thin areas have been grouped together.